The present invention relates generally to a telephoto zoom lens, and more particularly to a telephoto zoom lens system of high image quality yet large aperture ratio.
A grave problem with a zoom lens system of large aperture ratio and with an increase quantity of center light is that it is difficult to correct various aberrations in the process wherein the quantity of light due to off-axis light flux is securely maintained to prevent a lowering of field illuminance, as is well-known in the art. For this reason, a zooming mode applicable to a zoom lens system of small aperture ratio sometimes cannot be used for making the aperture ratio large. In other words, relative aperture ratios in lens groups increase, resulting in an increase in the amount of aberrations introduced in the lens groups, and so the movement of the lens groups often gives rise to some considerable aberration variations.
The paraxial arrangement and lens configuration of a zoom lens system are considered to depend on the angle of field used. A telephoto zoom lens system of large aperture ratio, too, has some features worthy of attention. For instance, it is required to have a certain flatness of field in the wide-angle zone.
Remarkable chromatic aberration occurring in the telephoto zone must be well corrected because the state of correction thereof is linked directly with image quality.
Among zoom lens systems so far proposed in the art, those disclosed in JP-B 63-58324 and JP-A 5-215966 have gained some fame. These zoom lens systems are each made up of, in order from the object side, a first lens group having a positive refracting power, a second lens group having a negative refracting power, a third lens group having a positive refracting power and a fourth lens group having a positive refracting power; four lens groups in all. The incidental conditions are that the first, second and third lens groups constitute a substantially afocal unit and the total length of the lens system is kept constant upon focusing on the object point at infinity.
The object of these proposals is to achieve a telephoto zoom lens system of large aperture ratio. More specifically, the former, because of large aperture ratio, aims at achieving large telephoto ratio. The latter intends to eliminate or limit aberration variations during focusing. That is, an internal focusing mode due to the division of the first lens group is put forward. The internal focusing mode has some remarkable effect on correction of aberrations, but involves a problem that the lens system increases in size because it is required to allow for a separation between the focusing space and the compensating lens group.
U.S. Pat. No. 5,059,007, on the other hand, proposes to make a lens group for correction of aberration variations movable. In other words, this proposes to move the otherwise fixed image surface-compensating lens group in the form of a focusing lens group, but there is the same problem as mentioned above.
Also available is a zoom lens system basically made up of, in order from the object side, a first lens group having a positive refracting power, a second lens group having a zooming action and a negative refracting power and a third lens group having a positive refracting power and designed to correct an image surface displacement during zooming, said three groups constituting together an afocal unit, and a fourth lens group having a positive refracting power and an image-forming action. This zoom lens system is known to have stable performance and an increase in overall size as the aperture ratio becomes large.
However, paraxial constructions remain substantially unchanged while little or no improvement is introduced in the lens construction of each lens group.
Accordingly, an object of the present invention is to provide a solution to problems with the prior art modes, especially a performance problem and, hence, to provide a combined zooming and focusing mode.
In the prior art modes, there is some considerable drop of the ability to delineate images due to aberration variations upon focusing on a finite object point. One chief reason is that the third-order spherical aberration coefficient is remarkably undercorrected because the first lens group is moved toward the object side. Another possible reason is that higher-order spherical aberration coefficients of different signs are not well corrected.
To solve these problems, the internal focusing mode has been put forward, where separations between a plural lens groups are varied. For instance, in the above-mentioned lens group divided into an afocal sub-group and a positive sub-group, it is the latter positive sub-group that takes a focusing role. However, this focusing mode does not only render the configuration and weight of the lens arrangement complicate and heavy, but also offers a hard-to-solve production problem because the sensitivity of the first lens group with respect to a decentering production error due to its tilt is high.
Correction of field distortion is important for the image quality of a telephoto zoom lens system, to say nothing of correction of chromatic aberration. To make the telephoto zoom lens system of large aperture ratio compact and enhance its image quality, it is also important in view of the properties of the optical system to make use of paraxial arrangement and lens construction that enable correction of curvature of field to be easily achieved. This is because, unlike a wide-angle lens system, the optical performance of its peripheral portion is also important for delineation wherein only an extended perspective sense of depth is enhanced.